Battery separator and battery using the same

ABSTRACT

The present invention relates to a battery separator including: a porous substrate; and a layer of a crosslinked polymer supported on at least one surface of the porous substrate, in which the crosslinked polymer is obtained by reacting (a) a reactive polymer having, in the molecule thereof, a reactive group containing active hydrogen with (b) a polycarbonate urethane prepolymer terminated by an isocyanate group.

TECHNICAL FIELD

The present invention relates to a battery separator and a battery usingthe separator. More particularly, the invention relates to a batteryseparator obtained by supporting a layer of a crosslinked polymer havinga polycarbonate urethane framework on a porous substrate, and relates toa battery using the separator.

BACKGROUND ART

Lithium ion secondary batteries having a high energy density areextensively used in recent years as power sources for small portableelectronic appliances such as cell phones and notebook type personalcomputers. Such a lithium secondary battery is produced through thesteps of stacking or winding sheet-form positive and negative electrodestogether with, for example, a porous polyolefin resin film, introducingthe resultant stack into a battery container constituted of, forexample, a metallic can, subsequently pouring an electrolyte solutioninto the battery container, and tightly sealing the opening of thebattery container.

Recently, however, such small portable electronic appliances areexceedingly strongly desired to be further reduced in size and weight.Under these circumstances, lithium ion secondary batteries also aredesired to be further reduced in thickness and weight. Batterycontainers of the laminated-film type have also come to be used in placeof conventional metallic can cases.

Compared with the conventional metallic can cases, battery containers ofthe laminated-film type have a drawback that an areal pressure formaintaining electrical connection between the separator and eachelectrode cannot be sufficiently applied to electrode surfaces. Becauseof this, these battery containers have a problem that the distancebetween the electrodes partly increases with the lapse of time due tothe expansion/contraction of the electrode active materials duringbattery charge/discharge, resulting in an increase in the internalresistance of the battery and hence in a decrease in batterycharacteristics. In addition, there is a problem that unevenness ofresistance occurs in the battery and this also reduces batterycharacteristics.

In the case of producing a sheet-form battery having a large area, therehas been a problem that the distance between the electrodes cannot bekept constant and the internal resistance of the battery becomes uneven,making it impossible to obtain sufficient battery characteristics.

In order to overcome such problems, it has been proposed to bondelectrodes to a separator with an adhesive resin layer constituted of amixed phase including an electrolyte-solution phase, a polymer gel layercontaining the electrolyte solution, and a solid polymer phase (see, forexample, patent document 1). Furthermore, a method has been proposedwhich includes coating a separator with a binder resin solutioncontaining a poly(vinylidene fluoride) resin as a main component,subsequently stacking electrodes on the coated separator, drying thebinder resin solution to form an electrode stack, introducing theelectrode stack into a battery container, and then pouring anelectrolyte solution into the battery container to obtain a battery inwhich the separator has been adhered to the electrodes (see, forexample, patent document 2).

It has also been proposed to obtain a battery containing electrodesadhered to a separator, by bonding a separator impregnated with anelectrolyte solution to positive and negative electrodes with a porousadhesive resin layer to bring the separator into close contact with theelectrodes and cause the adhesive resin layer to hold the electrolytesolution in the through-holes thereof (see, for example, patent document3).

However, those processes have had the following problem. The thicknessof the adhesive resin layer must be increased in order to obtainsufficient adhesive force between the separator and each electrode.Because of this and because the amount of the electrolyte solutionrelative to that of the adhesive resin cannot be increased, theresultant battery has increased internal resistance. Consequently,sufficient cycle characteristics and sufficient high-rate dischargecharacteristics cannot be obtained.

Furthermore, in the battery in which the separator has been adhered tothe electrodes with an adhesive resin as described above, the adhesivestrength between the separator and each electrode decreases when thebattery is placed in a high-temperature environment. As a result, thereis a concern that the separator might thermally contract to causeshort-circuiting between the electrodes. In addition, although theadhesive resin in the battery is in the state of being swollen with theelectrolyte solution, the adhesive resin layer has high internalresistance because electrolyte ions are less apt to diffuse in theadhesive resin than in the electrolyte solution, and exerts adverseinfluences on battery characteristics.

On the other hand, with respect to porous substrates for use as batteryseparators, various production processes have been known hitherto. Oneknown process is to produce a sheet made of, for example, a polyolefinresin and stretch the sheet at a high ratio (see, for example, patentdocument 4). However, the battery separator constituted of such a porousfilm obtained through high-ratio stretching has a problem that theseparator considerably contracts in high-temperature environments, suchas the case where a battery has undergone abnormal heating due tointernal short-circuiting, etc., and in some cases, comes not tofunction as a partition between the electrodes.

Consequently, to reduce the degree of heat shrinkage of batteryseparators which occurs in such a high-temperature environment isregarded as an important subject for improving the safety of batteries.In this respect, a process for producing a porous film for use as abattery separator in order to inhibit the heat shrinkage of batteryseparators occurring in high-temperature environments is, for example,known. This process includes melt-kneading ultrahigh-molecularpolyethylene together with a plasticizer, extruding the mixture througha die into a sheet form, and then extracting and removing theplasticizer to produce the porous film (see patent document 5). However,this process, in contrast to the method described above, has a problemthat the porous film obtained has insufficient strength because thisfilm has not undergone stretching.

Moreover, an attempt is recently being made to heighten the chargevoltage of batteries as one measure in increasing the capacity ofbatteries. However, to thus heighten the charge voltage, on the otherhand, poses a problem that a large amount of lithium is deintercalatedfrom composite oxides of lithium and cobalt or nickel, which aregenerally used as positive-electrode active materials, to bring thesecomposite oxides into a higher degree of oxidized state having higherreactivity. As a result, the separator, in particular, deterioratesconsiderably, resulting in battery performance deterioration.

In order to overcome such a problem, it has been proposed to form aporous layer of a fluororesin such as a polytetrafluoroethylene resinbetween a separator and a positive electrode (see patent document 6).For example, there is a statement therein to the effect that a preferredmethod for forming a porous polytetrafluoroethylene resin layer is tospray a suspension of a polytetrafluoroethylene resin on a separator anddry the suspension. However, the layer obtained using this method has anincreased thickness to sacrifice battery capacity although rich inporosity. In addition, use of this separator necessitates a large amountof an electrolyte solution.

Patent Document 1: JP-A-09-161814

Patent Document 2: JP-A-11-329439

Patent Document 3: JP-A-10-172606

Patent Document 4: JP-A-09-012756

Patent Document 5: JP-A-05-310989

Patent Document 6: JP-A-2007-157459

DISCLOSURE OF THE INVENTION

The invention has been achieved in order to overcome the variousproblems described above concerning battery separators. An object of theinvention is to provide a battery separator which has, in particular,excellent oxidation resistance and further has adhesiveness toelectrodes. Another object thereof is to provide a battery using such abattery separator.

Namely, the present invention relates to the following items (1) to (9).

-   (1) A battery separator including:

a porous substrate; and

a layer of a crosslinked polymer supported on at least one surface ofthe porous substrate,

in which the crosslinked polymer is obtained by reacting

(a) a reactive polymer having, in the molecule thereof, a reactive groupcontaining active hydrogen with

(b) a polycarbonate urethane prepolymer terminated by an isocyanategroup.

-   (2) The battery separator according to (1), in which the reactive    group containing active hydrogen is at least one kind selected from    a hydroxy group, a carboxyl group and an amino group.-   (3) The battery separator according to (1), in which the porous    substrate is a porous polyolefin resin film.-   (4) The battery separator according to (3), in which the porous    polyolefin resin film is a porous polyethylene resin film.-   (5) An electrode/separator laminate including:

the separator according to any one of (1) to (4); and

a positive electrode and a negative electrode laminated together withthe separator interposed therebetween,

in which at least one of the positive electrode and the negativeelectrode is adhered to the porous substrate by the crosslinked polymer.

-   (6) A battery including the electrode/separator laminate according    to (5).-   (7) The battery according to (6), in which the battery further    includes a nonaqueous electrolyte solution and the layer of the    crosslinked polymer faces at least the positive electrode.-   (8) A process for producing a battery, the process including:

stacking a positive electrode and a negative electrode together with theseparator according to any one of (1) to (4) interposed therebetween;

introducing the resultant stack into a battery container, followed bypouring a nonaqueous electrolyte solution into the battery container;and

forming an electrode/separator laminate in which at least one of thepositive electrode and the negative electrode is adhered to the poroussubstrate by the crosslinked polymer.

-   (9) The process for producing a battery according to (8), in which    the positive electrode and the negative electrode are stacked    together with the separator interposed therebetween so that the    layer of the crosslinked polymer faces at least the positive    electrode.

The battery separator of the invention is obtained by supporting on aporous substrate a layer of a crosslinked polymer obtained by reacting areactive polymer having in the molecule thereof a reactive groupcontaining active hydrogen with a polycarbonate urethane prepolymerterminated by an isocyanate group. The crosslinked polymer hence hasexcellent oxidation resistance and further has adhesiveness toelectrodes.

Consequently, this battery separator can be adhered to electrodes bystacking the electrodes on the separator to obtain anelectrode/separator stack; introducing the stack into a batterycontainer; and subsequently pouring a nonaqueous electrolyte solutioninto the battery container to cause at least part of the crosslinkedpolymer on the porous substrate to swell around the interface betweenthe crosslinked polymer and the electrode(s) and to penetrate into theelectrode active material(s) together with the electrolyte solution. Asa result, a battery having an electrode/separator laminate can beobtained.

Since the crosslinked polymer has a crosslinked structure, this polymerdoes not excessively dissolve or diffuse in the electrolyte solutionwhen swelled in the electrolyte solution. The crosslinked polymer doesnot exert an adverse influence on the electrolyte solution.

Furthermore, since the crosslinked polymer is a polymer obtained bycrosslinking a reactive polymer with a polycarbonate urethane prepolymerterminated by an isocyanate group and includes a polycarbonatestructure, the crosslinked polymer has high oxidation resistance.Because of this, the battery separator of the invention supporting alayer of this crosslinked polymer has high resistance to the highlyoxidizing environment present at the interface between the separator andthe positive electrode. Thus, a battery having a high energy density andexcellent charge/discharge characteristics can be provided.

BEST MODE FOR CARRYING OUT THE INVENTION

(Porous Substrate)

The porous substrate to be used in the invention is preferably onehaving a thickness in the range of 3 to 50 μm. In case where the poroussubstrate has a thickness less than 3 μm, the porous substrate hasinsufficient strength and there is a concern that use of this poroussubstrate as a separator in a battery might result in internalshort-circuiting between the electrodes. On the other hand, in casewhere the porous substrate has a thickness exceeding 50 μm, the batteryusing such a porous substrate as the separator has too large a distancebetween the electrodes, resulting in excessively high internalresistance of the battery.

The porous substrate to be use may be one which has pores having anaverage pore diameter of 0.01 to 5 μm and has a porosity in the range of20 to 95%. The porosity thereof is preferably in the range of 30 to 90%,most preferably in the range of 35 to 85%. In case where the poroussubstrate has too low a porosity, use of this porous substrate as abattery separator results in a decrease in the amount of ion conductionpaths, making it impossible to obtain sufficient batterycharacteristics. On the other hand, in case where the porous substratehas too high a porosity, this porous substrate has insufficient strengthwhen used as a battery separator. Such a porous substrate must be thickfrom the standpoint of obtaining required strength. This is undesirablebecause the internal resistance of the battery increases.

Furthermore, the porous substrate to be use may be one which has an airpermeability of 1,500 sec/100 cc or lower, preferably 1,000 sec/100 ccor lower. In case where the air permeability thereof is too high, thisporous substrate has low ionic conductivity when used as a batteryseparator, making it impossible to obtain sufficient batterycharacteristics. With respect to the strength of the porous substrate,it is preferred that the porous substrate should have a puncturestrength of 1 N or higher. This is because in case where the puncturestrength thereof is lower than 1 N, there is a concern that this poroussubstrate might break when an areal pressure is applied to between theelectrodes, resulting in internal short-circuiting.

It is preferred that the porous substrate should have a high affinityfor the reactive polymer which will be described later. It is thereforepreferred that when the porous substrate is constituted of a lowly polarmaterial, the surface thereof should be subjected to a suitablesurface-hydrophilizing treatment, such as a corona treatment, in orderto improve the affinity for the reactive polymer.

According to the invention, the porous substrate is not particularlylimited so long as it has the properties described above. However, whensolvent resistance and strength are taken into account, porous filmsmade of polyolefin resins such as polyethylene and polypropylene aresuitable. However, a porous polyethylene resin film is especiallysuitable because this resin has the property of melting upon heating toclose the pores and, as a result, can impart the so-called shutdownfunction to the battery. Examples of the polyethylene resin include notonly ethylene homopolymers but also copolymers of ethylene and anα-olefin such as propylene, butene, or hexene.

In particular, according to the invention, a porous film obtained fromultrahigh-molecular polyethylene as polyethylene is suitable for use asthe porous substrate. The term “ultrahigh-molecular polyethylene” meanspolyethylene having a weight-average molecular weight of 500,000 orhigher, preferably in the range of 500,000 to 3,000,000, and variouscommercial products are available. A mixture of ultrahigh-molecularpolyethylene and another resin may be formed into a porous film in orderto enhance the moldability of ultrahigh-molecular polyethylene and theadhesiveness of the porous film obtained.

According to the invention, paper also can be used as the poroussubstrate besides porous films of polytetrafluoroethylene and ofpolyimides, polyesters, polycarbonates, regenerated cellulose, etc.Furthermore, such porous films containing an inorganic filler, e.g.,silica, titanium oxide, alumina, or kaolinite, or a mineral filler,e.g., montmorillonite, dispersed therein can also be used as the poroussubstrate.

(Reactive Polymer)

The term “reactive polymer” in the invention means a polymer having inthe molecule thereof a reactive group containing active hydrogen. Theterm “reactive group containing active hydrogen” means a group which hasreactivity with an isocyanate group by the action of active hydrogen.Examples of such a reactive group include at least one kind selectedfrom a hydroxy group, carboxyl group, and amino group.

As will be described later, a crosslinked polymer having a polycarbonateurethane framework can be obtained according to the invention byreacting such a reactive polymer with a polycarbonate urethaneprepolymer terminated by an isocyanate group. The polycarbonate urethaneprepolymer terminated by an isocyanate group can be obtained by reactinga polycarbonate diol with a polyfunctional isocyanate.

Preferably, the reactive polymer can be obtained by subjecting a firstradical-polymerizable monomer, which has the reactive group, and asecond radical-polymerizable monomer, which does not have such areactive group, to radical copolymerization using a radicalpolymerization initiator.

The radical-polymerizable monomer having a reactive group is used in anamount in the range of 0.1 to 10% by weight, preferably 0.5 to 5% byweight, of the total monomer amount. In case where the amount of theradical-polymerizable monomer having a reactive group is smaller than0.1% by weight of the total monomer amount, the resultant reactivepolymer, when reacted with the isocyanate-terminated polycarbonateurethane prepolymer, which will be described later, gives a crosslinkedpolymer having too low an insoluble content. When an electrode/separatorstack is immersed in an electrolyte solution, this crosslinked polymeris not sufficiently inhibited from dissolving or diffusing in theelectrolyte solution because of the too low insoluble content, anddissolves and diffuses in an increased amount. As a result, adhesionbetween the porous substrate and the electrode(s) cannot be maintained,and there is a concern that battery deterioration might be acceleratedby the impurities. However, in case where the amount of theradical-polymerizable monomer having a reactive group is larger than 10%by weight, the resultant crosslinked prepolymer has too high a crosslinkdensity. This crosslinked polymer is excessively dense and does notsufficiently swell when in contact with an electrolyte solution. As aresult, an electrode/separator laminate cannot be obtained, and abattery having excellent characteristics cannot be obtained.

Examples of the first radical-polymerizable monomer, which has areactive group, are as follows. Examples of the monomer in which thereactive group is a carboxyl group include (meth)acrylic acid, itaconicacid, and maleic acid. Examples of the monomer in which the reactivegroup is a hydroxy group include hydroxyalkyl (meth)acrylates such as2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate,4-hydroxybutyl (meth)acrylate, and 6-hydroxyhexyl (meth)acrylate,(poly)alkylene glycol mono(meth)acrylates such as ethylene glycolmono(meth)acrylate, diethylene glycol mono(meth)acrylate, triethyleneglycol mono(meth)acrylate, hexaethylene glycol mono(meth)acrylate,propylene glycol mono(meth)acrylate, dipropylene glycolmono(meth)acrylate, tripropylene glycol mono(meth)acrylate, andpentapropylene glycol mono(meth)acrylate, and products of thering-opening addition of 2-hydroxyethyl (meth)acrylate withγ-butyrolactone. Examples of the monomer in which the reactive group isan amino group include products of the 1:1 reaction of a diamine with(meth)acryloyloxyethyl isocyanate.

The term “(meth)acrylic acid” means acrylic acid or methacrylic acid,“(meth)acrylate” means acrylate or methacrylate, and “(meth)acryloyloxy”means acryloyloxy or methacryloyloxy.

On the other hand, preferred examples of the secondradical-polymerizable monomer, which has no reactive group, include(meth)acrylates represented by general formula (I):

in which R₁ represents a hydrogen atom or a methyl group; A representsan oxyalkylene group having 2 or 3 carbon atoms (preferably, oxyethyleneor oxypropylene); R₂ represents an alkyl group having 1 to 6 carbonatoms or a fluoroalkyl group having 1 to 6 carbon atoms; and nrepresents an integer of 0 to 12;and vinyl esters represented by general formula (II):

in which R₃ represents a methyl group or an ethyl group, and R₄represents a hydrogen atom or a methyl group.

Examples of the (meth)acrylates represented by general formula (I)include methyl (meth)acrylate, ethyl (meth)acrylate, propyl(meth)acrylate, butyl (meth)acrylate, 2,2,2-trifluoroethyl(meth)acrylate, and 2,2,3,3-tetrafluoropropyl (meth)acrylate.

Besides the examples given above, examples of the (meth)acrylatesrepresented by general formula (I) include the following:

in which R₅ represents a hydrogen atom or a methyl group, and n is aninteger of 0 to 12.

Examples of the vinyl esters represented by general formula (II) includevinyl acetate and vinyl propionate.

As described above, the reactive polymer can be obtained by subjectingthe first radical-polymerizable monomer, which has a reactive group, andthe second radical-polymerizable monomer, which has no reactive group,to radical copolymerization using a radical polymerization initiator.This radical copolymerization may be conducted using any polymerizationtechnique selected from solution polymerization, bulk polymerization,suspension polymerization, emulsion polymerization, and the like.However, it is preferred to employ solution polymerization or suspensionpolymerization from the standpoints of ease of polymerization,molecular-weight regulation, post-treatment, etc.

The radical polymerization initiator is not particularly limited. Forexample, use may be made of N,N′-azobisisobutyronitrile, dimethylN,N′-azobis(2-methylpropionate), benzoyl peroxide, lauroyl peroxide, orthe like. In this radical copolymerization, a molecular weight regulatorsuch as a mercaptan can be used according to need.

In the invention, the reactive polymer preferably has a weight-averagemolecular weight of 10,000 or higher. In case where the weight-averagemolecular weight of the reactive polymer is lower than 10,000, thecrosslinked polymer obtained therefrom is less apt to swell with anelectrolyte solution and this reduces the characteristics of the batteryto be obtained. On the other hand, the upper limit of the weight-averagemolecular weight of the reactive polymer is not particularly limited.However, the upper limit thereof may be about 3,000,000, preferablyabout 2,500,000, from the standpoint that the crosslinked polymer to beobtained therefrom can retain an electrolyte solution as a gel. Inparticular, it is preferred according to the invention that the reactivepolymer should have a weight-average molecular weight in the range of100,000 to 2,000,000.

(Polycarbonate Urethane Prepolymer Terminated by Isocyanate Group)

The polycarbonate urethane prepolymer terminated by an isocyanate group(hereinafter referred to simply as urethane prepolymer) in the inventionis an oligomer preferably obtained by reacting an aliphaticpolycarbonate diol with a polyfunctional isocyanate in such a proportionthat the molar ratio of the isocyanate groups possessed by thepolyfunctional isocyanate to the hydroxy groups possessed by thepolycarbonate diol (hereinafter referred to as NCO/OH molar ratio) isgenerally in the range of 1.2 to 3.3, preferably in the range of 1.5 to2.5. Although the molecular weight of the urethane prepolymer to beobtained changes with the NCO/OH molar ratio, a urethane prepolymer inwhich both ends of the molecule each substantially are an isocyanategroup can be obtained when the NCO/OH molar ratio is regulated so as tobe within that range.

As is already well known, the aliphatic polycarbonate diol can beobtained, for example, by reacting an aliphatic diol with phosgene or bythe ring-opening polymerization of an alkylene carbonate. In the case ofobtaining the polycarbonate diol by reacting an aliphatic diol withphosgene, the aliphatic diol to be used is not particularly limited.Examples thereof include ethylene glycol, diethylene glycol, propyleneglycol, dipropylene glycol, trimethylene glycol, 1,4-tetramethylenediol,1,5-pentamethylenediol, neopentyl glycol, 1,6-hexamethylenediol, and1,4-cyclohexanediol. These aliphatic diols may be used alone or incombination of two or more thereof.

In the use of obtaining the polycarbonate diol by the ring-openingpolymerization of an alkylene carbonate, the alkylene carbonate to beused is also not particularly limited. Examples thereof include ethylenecarbonate, trimethylene carbonate, tetramethylene carbonate, andhexamethylene carbonate. These alkylene carbonates also may be usedalone or as a mixture of two or more thereof.

The aliphatic polycarbonate diol can be obtained also by reacting analkylene carbonate, such as those shown above, or a dialkyl carbonatewith the aliphatic diol. Examples of the dialkyl carbonate includedimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, anddi-n-butyl carbonate.

According to the invention, the aliphatic polycarbonate diol to be usedpreferably has repeating units represented by general formula (III):

in which R represents an aliphatic diol residue having 2 to 6 carbonatoms. In the repeating units represented by general formula (III),however, the aliphatic diol residues, i.e., alkylene groups, of R in therespective repeating units may differ in the number of carbon atoms.

For example, according to the invention, the aliphatic polycarbonatediol to be used may be one having repeating units represented by generalformula (IIIa) and general formula (IIIb):

in which Ra and Rb each represent an aliphatic diol residue having 2 to6 carbon atoms, but differ from each other in the number of carbonatoms.

The aliphatic diol residue having 2 to 6 carbon atoms, as describedabove, is the aliphatic hydrocarbon group contained in an aliphatic diolsuch as, for example, ethylene glycol, 1,3-trimethylenediol,1,4-tetramethylenediol, 1,5-pentamethylenediol, neopentyl glycol,1,6-hexamethylenediol, or 1,4-cyclohexanediol, and preferably is alinear or branched alkylene group.

On the other hand, as the polyfunctional isocyanate, use can be made ofan aromatic, araliphatic, alicyclic, or aliphatic diisocyanate such as,for example, phenylene diisocyanate, tolylene diisocyanate,diphenylmethane diisocyanate, diphenyl ether diisocyanate, hexamethylenediisocyanate, or cyclohexane diisocyanate. Besides these, use may bemade of a so-called isocyanate adduct obtained by causing a polyol suchas trimethylolpropane to add any of these diisocyanates.

(Crosslinked Polymer and Battery Separator)

According to the invention, the reactive polymer is reacted with theurethane prepolymer to thereby crosslink the reactive polymer with theprepolymer. Thus, a crosslinked polymer having a polycarbonate urethaneframework can be obtained. The battery separator according to theinvention is one obtained by supporting a layer of such a crosslinkedpolymer on the porous substrate. Namely, the battery separator accordingto the invention includes a porous substrate and a layer of thecrosslinked polymer supported thereon.

According to the invention, to support a layer of the crosslinkedpolymer on at least one surface of a porous substrate may suffice forthe function of the desired battery separator. Furthermore, it ispossible to support a layer of the crosslinked polymer not only as acontinuous layer but also in any of various arrangements.

For supporting a crosslinked polymer on a porous substrate, use may bemade, for example, of a method including dissolving the reactive polymerand the urethane prepolymer in a suitable solvent, e.g., acetone, ethylacetate, butyl acetate, or toluene, applying the resultant solution to aporous substrate by a suitable technique, e.g., casting or spraycoating, and subsequently reacting the reactive polymer with theurethane prepolymer to crosslink the reactive polymer either afterheating the coating to remove the solvent used or while heating thecoating to remove the solvent used.

Another method may be used, in which a solution containing the reactivepolymer and the urethane prepolymer is applied to a release sheet anddried to form on the release sheet a thin layer containing a mixture ofthe reactive polymer and the urethane prepolymer. Thereafter, thisrelease sheet is superposed on a porous substrate, and the stack isheated and pressed to transfer the thin layer of a mixture of thereactive polymer and the urethane prepolymer to the porous substrate.Subsequently, the thin layer of a mixture of the reactive polymer andthe urethane prepolymer on the porous substrate is heated to react thereactive polymer with the urethane prepolymer and crosslink the reactivepolymer. In either method, heating at 90° C. for 48 hours, for example,suffices for reacting the reactive polymer with the urethane prepolymer.

Furthermore, the above-mentioned methods each may be conducted in thefollowing manner. A solution containing the reactive polymer and theurethane prepolymer is prepared and heated to partially react andcrosslink the reactive polymer with the urethane prepolymer beforehandto such a degree that the crosslinked polymer yielded does not undergophase separation in the solution. Thereafter, this solution is appliedto a porous substrate or a release sheet, heated to remove the solvent,and further heated to react the reactive polymer with the urethaneprepolymer and crosslink the reactive polymer.

It is typically preferred that the release sheet to be used should be apolypropylene resin sheet. However, the release sheet is not limitedthereto, and usable sheets include, for example, sheets of poly(ethyleneterephthalate), polyethylene, vinyl chloride, engineering plastics, andthe like, paper (in particular, resin-impregnated paper), syntheticpapers, laminates of these, and the like. These sheets may haveundergone a back treatment with a compound such as a silicone orlong-chain alkyl compound according to need.

When a layer of the crosslinked polymer having a polycarbonate urethaneframework formed by reacting the reactive polymer with the urethaneprepolymer is supported on a porous substrate, the proportions of thereactive polymer and urethane prepolymer to be used depend not only onthe amount of reactive groups contained in the reactive polymer and theamount of isocyanate groups contained in the urethane prepolymer butalso on the properties, e.g., molecular weights, of the reactive polymerand urethane prepolymer. Usually, however, the proportion of theurethane prepolymer is in the range of 10 to 150 parts by weight per 100parts by weight of the reactive polymer. In case where the proportion ofthe urethane prepolymer per 100 parts by weight of the reactive polymeris smaller than 10 parts by weight, the crosslinked polymer obtaineddoes not have satisfactory oxidation resistance. On the other hand, incase where the proportion of the urethane prepolymer per 100 parts byweight of the reactive polymer is larger than 150 parts by weight, thecrosslinked polymer obtained has too high a crosslink density. Even whena porous substrate having such a crosslinked polymer supported thereonis used for battery production, a battery having excellentcharacteristics cannot be obtained.

In the invention, the amount of the reactive polymer and urethaneprepolymer to be supported on a porous substrate, i.e., the amount ofthe crosslinked polymer to be supported on a porous substrate, isgenerally in the range of 0.2 to 5.0 g/m², preferably in the range of0.3 to 3.0 g/m², although the amount thereof depends on the kinds of thereactive polymer and urethane prepolymer used and on the manner in whichthese ingredients are supported on the porous substrate. In case wherethe amount of the crosslinked polymer supported on the porous substrateis too small, the separator obtained does not adhere to an electrode atsufficient adhesive force. Conversely, too large amounts thereof areundesirable because the battery using the separator thus obtained hasreduced characteristics.

According to the invention, the crosslinked polymer obtained by reactingthe reactive polymer with the urethane prepolymer has an insolublecontent in the range of 50 to 99% by weight, preferably 60 to 99% byweight, more preferably 70 to 99% by weight. The term “insolublecontent” herein means the proportion of the crosslinked polymerremaining on a porous substrate after the porous substrate having thecrosslinked polymer supported thereon is immersed in ethyl acetate atroom temperature with stirring for 6 hours.

(Battery)

Electrodes are stacked together with the separator according to theinvention obtained by the method described above. For example, apositive electrode and a negative electrode are stacked together withthe separator according to the invention interposed therebetween.According to need, the stack is pressed with heating to conductpress-bonding. Thus, the electrodes are temporarily adhered and stackedto the separator, and an electrode/separator stack can be obtained.

The electrodes, i.e., the negative electrode and positive electrode, tobe used in the invention generally are sheet electrodes obtained bysupporting an active material optionally together with a conductivematerial on a conductive substrate by a resin binder, although thisvaries depending on the battery.

In the invention, the electrode/separator stack is not limited so longas electrodes have been stacked together with the separator.Consequently, the electrode/separator stack to be used is, for example,one for obtaining a negative electrode/separator/positive electrodeconfiguration, a negative electrode/separator/positiveelectrode/separator configuration, or the like according to thestructure and shape of the battery. The electrode/separator stack may bein a sheet form or may have been wound.

Battery production using the separator according to the invention isexplained. As described above, electrodes are stacked or wound togetherwith the separator and temporarily adhered thereto to obtain anelectrode/separator stack. Subsequently, this stack is introduced into abattery container constituted of a metallic can, laminated film, or thelike, and terminal welding or the like is conducted if necessary.Thereafter, a given amount of a nonaqueous electrolyte solution ispoured into the battery container, and the opening of this batterycontainer is tightly sealed. At least part of the crosslinked polymersupported on the separator is swelled in the electrolyte solution aroundthe interface between the separator and each electrode and caused topenetrate into interstices of the electrode active material. Thecrosslinked polymer is thereby caused to produce an anchoring effect onboth the porous substrate and the electrode. An electrode/separatorlaminate in which the electrodes have been adhered to the poroussubstrate by the action of the crosslinked polymer can be therebyobtained. Thus, a battery having the electrode/separator laminate can beobtained.

According to the invention, since the crosslinked polymer supported onthe porous substrate has a high insoluble content as described above,the crosslinked polymer, when immersed in an electrolyte solution inbattery fabrication, is inhibited from dissolving or diffusing in theelectrolyte solution. Consequently, the crosslinked polymer, when usedin battery production, rarely dissolves in the electrolyte solution toreduce battery characteristics.

In general, the wettability of the electrodes by the electrolytesolution is dramatically improved by initial charge/discharge.Simultaneously with the improvement in wettability, the crosslinkedpolymer swelled with the electrolyte solution further penetrates intothe interstices of the electrode active materials to render theseparator/electrode adhesion more tenacious.

According to the invention, by introducing the electrode/separator stackinto a battery container, pouring an electrolyte solution into thebattery container, and then heating the stack, the crosslinked polymersupported on the porous substrate can be brought into closer contactwith the electrodes. Conditions for the heating may generally include atemperature of 40 to 100° C. and a period of about 0.5 to 24 hours,although the heat resistance of the materials constituting the batteryand productivity should also be taken into account.

In the battery separator according to the invention, the crosslinkedpolymer, on one hand, functions as an adhesive for adhering an electrodeto the separator, as described above, and is useful for forming anelectrode/separator laminate. By thus forming an electrode/separatorlaminate, the battery can be inhibited from suffering a trouble that anelectrode slides on the separator and is exposed or a trouble that theseparator contracts to expose an electrode.

In particular, according to the invention, the separator in the batteryobtained is adherent to the electrodes. Consequently, even when thebattery is placed in a high-temperature environment such as, forexample, a 150° C. environment, the separator (strictly speaking, theporous substrate) has a low degree of areal heat shrinkage, which isgenerally 20% or lower, preferably 15% or lower.

As described above, the mode in which a layer of the crosslinked polymeris supported on a porous substrate is not particularly limited accordingto the invention so long as the crosslinked polymer satisfies thefunction as an adhesive. Consequently, a layer of the crosslinkedpolymer may be supported on the whole area of a surface of the poroussubstrate or, in some cases, may be supported partly on the surface, forexample, partly supported in a stripe, spot, lattice, streak, orhoneycomb arrangement. Furthermore, a layer of the crosslinked polymermay be supported only on one surface of a porous substrate or on bothsurfaces thereof.

Moreover, in the battery separator according to the invention, thecrosslinked polymer has high oxidation resistance because thecrosslinked polymer has a crosslinked structure formed by crosslinkingthe reactive polymer using the urethane prepolymer as a crosslinkingagent as described above and has a polycarbonate framework.Consequently, the separator according to the invention, on another hand,has the function of imparting high oxidation resistance to the poroussubstrate constituting the separator, and is useful. Especially when theseparator substrate is a porous film of a polyolefin resin such aspolyethylene or polypropylene, use of a heightened charge voltage makesthe positive-electrode active material have a highly oxidized state andhigh oxidation reactivity and, hence, the separator is apt to be damagedand deteriorated, as described above. In such cases, however, by usingthe separator constituted of a porous polyolefin resin film having alayer of the crosslinked polymer supported thereon to form anelectrode/separator laminate so that the layer of the crosslinkedpolymer is located on the positive-electrode side, excellent oxidationresistance can be imparted to the separator. Thus, a battery having ahigh energy density and excellent charge/discharge characteristics canbe obtained.

The nonaqueous electrolyte solution is a solution prepared by dissolvingan electrolyte salt in a suitable organic solvent. As the electrolytesalt, use can be made of salts composed of: a cation component which ishydrogen, an alkali metal such as lithium, sodium, or potassium, analkaline earth metal such as calcium or strontium, a tertiary orquaternary ammonium salt, or the like; and an anion component which isan inorganic acid such as hydrochloric acid, nitric acid, phosphoricacid, sulfuric acid, borofluoric acid, hydrofluoric acid,hexafluorophosphoric acid, or perchloric acid or an organic acid such asa carboxylic acid, organic sulfonic acid, or fluorine-substitutedorganic sulfonic acid. Especially preferred of these are electrolytesalts in which the cation component is an alkali metal ion.

Examples of such electrolytic salts in which the cation component is analkali metal ion include alkali metal perchlorates such as lithiumperchlorate, sodium perchlorate, and potassium perchlorate, alkali metaltetrafluoroborates such as lithium tetrafluoroborate, sodiumtetrafluoroborate, and potassium tetrafluoroborate, alkali metalhexafluorophosphates such as lithium hexafluorophosphate and potassiumhexafluorophosphate, alkali metal trifluoroacetates such as lithiumtrifluoroacetate, and alkali metal trifluoromethanesulfonates such aslithium trifluoromethanesulfonate.

Especially in the case of obtaining a lithium ion secondary batteryaccording to the invention, suitable examples of the electrolyte salt tobe used are lithium hexafluorophosphate, lithium tetrafluoroborate,lithium perchlorate, and the like.

The solvent to be used for the electrolyte salt in the invention can beany solvent in which the electrolyte salt dissolves. Usable nonaqueoussolvents include cyclic esters such as ethylene carbonate, propylenecarbonate, butylene carbonate, and γ-butyrolactone, ethers such astetrahydrofuran and dimethoxyethane, and chain esters such as dimethylcarbonate, diethyl carbonate, and ethyl methyl carbonate. These solventsmay be used alone or as a mixture of two or more thereof.

EXAMPLES

The invention will be explained below by reference to Examples. However,the invention should not be construed as being limited by the followingExamples in any way. In the following, properties of porous substratesand battery characteristics were evaluated in the manners shown below.

(Thickness of Porous Substrate)

The thickness of a porous substrate was determined through a measurementwith a 1/10,000 mm thickness gauge and based on a photograph of asection of the porous substrate taken with a scanning electronmicroscope at a magnification of 10,000.

(Porosity of Porous Substrate)

The porosity of a porous substrate was calculated using the followingequation from the weight W (g) per unit area S (cm²) of the poroussubstrate, the average thickness t (cm) thereof, and the density d(g/cm³) of the resin constituting the porous substrate.Porosity (%)=(1−(W/S/t/d))×100(Air Permeability of Porous Substrate)

The air permeability of a porous substrate was determined according toJIS P 8117.

(Puncture Strength of Porous Substrate)

A puncture test of a porous substrate was conducted using compressiontester KES-G5, manufactured by Kato Tech Co., LTD. The maximum load wasread from a load-deformation curve obtained in the measurement and wastaken as the puncture strength. A needle having a diameter of 1.0 mm anda radius of curvature of the tip of 0.5 mm was used to conduct the testat a rate of 2 mm/sec.

(Insoluble Content of Crosslinked Polymer)

A porous substrate having a crosslinked polymer supported thereon in aknown amount A was weighed to measure the weight B thereof.Subsequently, this porous substrate having the crosslinked polymersupported thereon was immersed in ethyl acetate at room temperature for6 hours and then air-dried. Thereafter, the thus-treated poroussubstrate having the crosslinked polymer supported thereon was weighedto measure the weight C thereof. The insoluble content of thecrosslinked polymer was calculated using the following equation.Insoluble content (%)=((A−(B−C))/A)×100

Reference Example 1

(Preparation of Electrode Sheets)

Eighty-five parts by weight of lithium cobalt oxide (Cellseed C-10,manufactured by Nippon Chemical Industrial Co., Ltd.) as apositive-electrode active material was mixed with 10 parts by weight ofacetylene black (Denka Black, manufactured by Denki Kagaku Kogyo K.K.)as a conduction aid material and 5 parts by weight of a vinylidenefluoride resin (KF Polymer L #1120, manufactured by Kureha ChemicalIndustry Co., Ltd.) as a binder. This mixture was slurried withN-methyl-2-pyrrolidone so as to result in a solid concentration of 15%by weight. This slurry was applied in a thickness of 200 μm to analuminum foil having a thickness of 20 μm (current collector) andvacuum-dried at 80° C. for 1 hour and then at 120° C. for 2 hours. Thecoated foil was pressed with a roller press to prepare apositive-electrode sheet having an active-material layer with athickness of 100 μm.

Eighty parts by weight of mesocarbon microbeads (MCMB 6-28, manufacturedby Osaka Gas Chemical Co., Ltd.) as a negative-electrode active materialwere mixed with 10 parts by weight of acetylene black (Denka Black,manufactured by Denki Kagaku Kogyo K.K.) as a conduction aid materialand 10 parts by weight of a vinylidene fluoride resin (KF Polymer L#1120, manufactured by Kureha Chemical Industry Co., Ltd.) as a binder.This mixture was slurried with N-methyl-2-pyrrolidone so as to result ina solid concentration of 15% by weight. This slurry was applied in athickness of 200 μm to a copper foil having a thickness of 20 μm(current collector) and dried at 80° C. for 1 hour and then dried at120° C. for 2 hours. The coated foil was pressed with a roller press toprepare a negative-electrode sheet having an active-material layer witha thickness of 100 μm.

Reference Example 2

(Production of Porous Polyethylene Resin Film)

Fifteen parts by weight of ultrahigh-molecular polyethylene having aweight-average molecular weight of 1,000,000 (melting point, 137° C.)was evenly mixed with 85 parts by weight of liquid paraffin to obtain aslurry. This slurry was melt-kneaded with a twin-screw extruder at atemperature of 170° C. and extruded through a coathanger die into asheet having a thickness of 2 mm. The resultant sheet was cooled whilebeing taken off with a roll. Thus, a gel sheet having a thickness of 1.3mm was obtained. This gel sheet was subjected at a temperature of 123°C. to simultaneous biaxial stretching in which the sheet was stretchedin the MD (machine direction) 4.5 times and in the TD (transversedirection) 5 times. Thus, a stretched film was obtained.

Decane was used to remove the liquid paraffin from the stretched film.Thereafter, the film was dried at room temperature to remove the decane.Thus, a porous film was obtained. The porous film obtained washeat-treated at 125° C. in air for 3 minutes to obtain a porouspolyethylene resin film. The porous film obtained had a thickness of 16μm, porosity of 39%, an air permeability of 270 sec/100 cc, and apuncture strength of 4 N.

Comparative Example 1

The negative-electrode sheet obtained in Reference Example 1, the porouspolyethylene resin film obtained in Reference Example 2, and thepositive-electrode sheet obtained in Reference Example 1 were stacked inthis order to obtain an electrode/porous film stack. This stack wasintroduced into an aluminum laminate package, and an electrolytesolution constituted of an ethylene carbonate/diethyl carbonate (1/1 byweight) mixed solvent containing lithium hexafluorophosphate dissolvedtherein in a concentration of 1.4 mol/L was poured into the package,which was then sealed. Thereafter, the resultant battery was charged ata current of 0.2 CmA until the voltage reached 3.5 V. Thus, a sealedlaminate type battery was obtained.

(Evaluation of Battery Characteristics)

The battery obtained was charged and discharged twice at roomtemperature at a current of 0.2 CmA. Thereafter, this battery wassubjected to tests for evaluating the following three items of batterycharacteristics. Separate batteries were respectively subjected to thetests for evaluating the following three items of batterycharacteristics.

(Rate Characteristics)

A battery was charged at 0.2 CmA and then discharged at 0.2 CmA todetermine the 0.2 CmA discharge capacity A. Subsequently, the batterywas charged at 0.2 CmA and then discharged at 2 CmA to determine the 2CmA discharge capacity B. The rate characteristics were calculated onthe basis of the following equation.Rate characteristics (%)=(2 CmA discharge capacity B)/(0.2 CmA dischargecapacity A)

Batteries which had been evaluated for rate characteristics in themanner described above were subjected to the following Determination ofDegree of Areal Shrinkage of Porous Substrate.

(Determination of Degree of Areal Shrinkage of Porous Substrate)

A battery which had been examined for rate characteristics, which arethe evaluation item of battery characteristics described above, wassandwiched between a pair of glass plates, and the opposed ends of thepair of glass plates were fixed with a polyimide tape in order toprevent the distance between the glass plates from enlarging. Thus, atest structure was assembled. This test structure was placed in 150° C.drying oven for 1 hour and then cooled. Subsequently, this teststructure was disassembled, and the porous substrate was peeled from theelectrode/crosslinked-polymer-supporting porous substrate laminateobtained. This porous substrate was read with a scanner and compared inarea with the porous substrate which had not been tested. Thus, thedegree of areal shrinkage of the porous substrate was determined.

(Continuous Charge Characteristics)

A battery was placed in a thermostatic chamber having a temperature of60° C. and subjected to constant-current constant-voltage charge at acurrent of 0.2 CmA and a voltage of 4.25 V. During charge at a currentof 0.2 CmA, when the battery voltage has reached 4.25 V, the currentvalue decreases. However, a phenomenon is observed in which the currentvalue which once decreased in that situation increases again. Thisphenomenon is thought to indicate that some chemical reaction is takingplace in the vicinity of the positive electrode, which is highly active,at a high voltage. Because of this, current behavior in the chargedescribed above was examined for 7 days as an index for evaluating theoxidation resistance of the separator. When an increase in current valuewas observed in this examination, the time period from the initiation ofthe test to the point of time when the increase in current value wasobserved was measured. In the case where no increase in current valuewas observed in the 7-day examination, this battery is indicated by “noincrease”.

(High-Temperature Storage Test)

A battery was continuously subjected for 12 hours to constant-currentconstant-voltage charge at a current of 0.2 CmA and a voltage of 4.2 V.Subsequently, the battery in the fully charged state was stored in an80° C. thermosetting chamber for 4 days and then examined for batteryvoltage at a temperature of 80° C.

The results of the rate characteristics, continuous chargecharacteristics, and high-temperature storage test of the battery areshown in Table 1 together with the degree of areal shrinkage of theporous substrate in the battery.

Reference Example 3

(Preparation of Reactive Polymer)

Into a three-necked flask having a capacity of 500 mL and equipped witha reflux condenser were introduced 84 g of methyl methacrylate, 2.0 g of4-hydroxybutyl acrylate, 14.0 g of 2-methoxyethyl acrylate, 25 g ofethyl acetate, and 0.20 g of N,N′-azobisisobutyronitrile. While nitrogengas was being introduced, the contents were stirred and mixed for 30minutes and then heated to 70° C. to initiate radical polymerization. Atthe time when about 1 hour had passed, an increase in the viscosity ofthe reaction mixture was observed. Thereafter, while ethyl acetate wasbeing added to the reaction mixture, the temperature was kept almostconstant and the polymerization was continued for further 8 hours.

After completion of the reaction, the reaction mixture obtained wascooled to 40° C., and ethyl acetate was added thereto. The contents werestirred and mixed until the mixture became wholly homogeneous. Thus, anethyl acetate solution of a reactive polymer (concentration, 15% byweight) was obtained.

Subsequently, 100 g of the polymer solution was added to 600 mL ofheptane with stirring with a high-speed mixer to precipitate thepolymer. The polymer was taken out by filtration, repeatedly washed withheptane several times, dried in air, and then vacuum-dried in adesiccator for 6 hours to obtain the reactive polymer as a white powder.

Comparative Example 2

Ten grams of the reactive polymer obtained in Reference Example 3 wasdissolved in ethyl acetate at room temperature to prepare areactive-polymer solution having a concentration of 10% by weight.Thereto was added 3.14 g of a polyfunctional isocyanate (hexamethylenediisocyanate/trimethylolpropane adduct; ethyl acetate solution; solidcontent 25%; Coronate HL, manufactured by Nippon Polyurethane Co.,Ltd.). The polyfunctional isocyanate was dissolved therein to prepare acoating fluid containing the reactive polymer and the polyfunctionalisocyanate.

This coating fluid was applied to one side of a polypropylene resinsheet with a wire-wound bar and then heated at 50° C. for 5 minutes tovolatilize the ethyl acetate. Thus, a thin layer constituted of amixture of the reactive polymer and the polyfunctional isocyanate wasformed on the polypropylene resin sheet.

This polypropylene resin sheet was superposed on the porous polyethyleneresin film obtained in Reference Example 2, so that the thin layerconstituted of a mixture of the reactive polymer and the polyfunctionalisocyanate was in contact with the porous film. The resultant stack waspassed through the nip between laminating rolls heated at a temperatureof 125° C. and was thereby heated and pressed to transfer the thin layerconstituted of a mixture of the reactive polymer and the polyfunctionalisocyanate to one side of the porous polyethylene resin film.

Subsequently, the stack composed of the porous polyethylene resin filmhaving the thin layer and the polypropylene resin sheet was heated at90° C. for 48 hours to react the reactive polymer with thepolyfunctional isocyanate and crosslink the reactive polymer, therebyforming a crosslinked polymer. Thereafter, the polypropylene resin sheetwas peeled off to obtain a porous polyethylene resin film having thecrosslinked polymer supported on one side thereof. The amount of thecrosslinked polymer supported on the porous polyethylene resin film was0.5 g/m².

For reasons of convenience, the weight of the crosslinked polymerpresent on the porous polyethylene resin film was taken as the weight ofthe thin layer constituted of a mixture of the reactive polymer and thepolyfunctional isocyanate and formed on the polypropylene resin sheet,and the amount of the crosslinked polymer supported on the porouspolyethylene resin film was determined in the following manner. Namely,a piece having a size of 5 cm×2 cm was cut out of the polypropyleneresin sheet on which the thin layer constituted of a mixture of thereactive polymer and the polyfunctional isocyanate had been formed, andthe weight A thereof was measured. Subsequently, the thin layerconstituted of a mixture of the reactive polymer and the polyfunctionalisocyanate was completely removed from the polypropylene resin sheet,and the weight B of this polypropylene resin sheet was thereaftermeasured. The amount of the crosslinked polymer on the porouspolyethylene resin film was calculated using (A−B)×1,000 (g/m²).

The negative-electrode sheet obtained in Reference Example 1, the porouspolyethylene resin film having the crosslinked polymer supportedthereon, and the positive-electrode sheet obtained in Reference Example1 were stacked in this order so that the crosslinked polymer on theporous film faced the positive-electrode sheet. Thus, anelectrode/crosslinked-polymer-supporting porous polyethylene resin filmstack was obtained. This stack was introduced into an aluminum laminatepackage, and an electrolyte solution constituted of an ethylenecarbonate/diethyl carbonate (1/1 by weight) mixed solvent containinglithium hexafluorophosphate dissolved therein in a concentration of 1.4mol/L was poured into the package, which was then sealed. Thereafter,the resultant battery was charged at a current of 0.2 CmA until thevoltage reached 3.5 V, and was then placed in a 50° C. thermostaticchamber for 24 hours to accelerate adhesion between the electrode sheetsand the separator. Thus, a sealed laminate type battery was obtained.

With respect to the battery obtained, the results of the ratecharacteristics, continuous charge characteristics, and high-temperaturestorage test of the battery are shown in Table 1 together with thedegree of areal shrinkage of the porous substrate, as in the case ofComparative Example 1.

Reference Example 4

(Preparation of Reactive Polymer)

Into a three-necked flask having a capacity of 500 mL and equipped witha reflux condenser were introduced 98 g of methyl methacrylate, 2.0 g of4-hydroxybutyl acrylate, 25 g of ethyl acetate, and 0.20 g ofN,N′-azobisisobutyronitrile. While nitrogen gas was being introduced,the contents were stirred and mixed for 30 minutes and then heated to70° C. to initiate radical polymerization. At the time when about 2hours had passed, an increase in the viscosity of the reaction mixturewas observed. Thereafter, while ethyl acetate was being added, thetemperature was kept almost constant and the polymerization wascontinued for further 8 hours.

After completion of the reaction, the reaction mixture obtained wascooled to 40° C., and ethyl acetate was added thereto. Thereafter, thecontents were stirred and mixed with heating until the mixture becamewholly homogeneous. Thus, a reactive-polymer solution (concentration,25% by weight) was obtained.

Subsequently, 100 g of the polymer solution was added to 600 mL ofheptane with stirring with a high-speed mixer to precipitate thepolymer. The polymer was taken out by filtration, repeatedly washed withheptane several times, dried in air, and then vacuum-dried in adesiccator for 6 hours to obtain the reactive polymer as a white powder.

(Preparation of Polycarbonate Urethane Prepolymer Terminated byIsocyanate Group)

While nitrogen gas was being introduced into a three-necked flask havinga capacity of 300 mL and equipped with a reflux condenser, 18.5 g of apoly(hexamethylene carbonate) diol (Nippollan 980R, manufactured byNippon Polyurethane Co., Ltd.) and 25.2 g of toluene were introducedinto the flask and stirred to dissolve the diol. Thereafter, a solutionprepared by mixing 4.98 g of poly(hexamethylene diisocyanate) (HDI;manufactured by Nippon Polyurethane Co., Ltd.) and 9.98 g of toluene wasmixed with that solution. After the solutions were stirred and mixedevenly, the resultant mixture was heated to 60° C. and reacted for 15hours. This mixture was cooled to room temperature, and 136.98 g oftoluene was further added thereto to obtain a toluene solution of anisocyanate-terminated polycarbonate urethane prepolymer having aconcentration of 12% by weight.

Example 1

Six grams of the reactive polymer obtained in Reference Example 4 wasdissolved in toluene at room temperature to prepare 50 g of areactive-polymer solution having a concentration of 12% by weight. Thissolution was mixed with 22.5 g of the toluene solution of anisocyanate-terminated polycarbonate urethane prepolymer having aconcentration of 12% by weight obtained in Reference Example 4, and theresultant mixture was stirred. Furthermore, 145 g of toluene was addedto the mixture solution obtained. Thus, a coating fluid having a solidconcentration of 4% by weight was prepared.

This coating fluid was applied to one side of a polypropylene resinsheet with a spin coater and then dried at 50° C. for 1 hour tovolatilize the toluene. Thus, a thin layer constituted of a mixture ofthe reactive polymer and the urethane prepolymer was formed on thepolypropylene resin sheet.

This polypropylene resin sheet was superposed on the porous polyethyleneresin film obtained in Reference Example 2, so that the thin layerconstituted of a mixture of the reactive polymer and the urethaneprepolymer was in contact with the porous film. The resultant stack waspassed through the nip between laminating rolls heated at a temperatureof 125° C. and was thereby heated and pressed to transfer the thin layerconstituted of a mixture of the reactive polymer and the urethaneprepolymer to one side of the porous polyethylene resin film.

Subsequently, the stack composed of the porous polyethylene resin filmhaving the thin layer and the polypropylene resin sheet was heated at90° C. for 48 hours to react the reactive polymer with the urethaneprepolymer and crosslink the reactive polymer, thereby forming acrosslinked polymer having an insoluble content of 99%. Thereafter, thepolypropylene resin sheet was peeled off to obtain a porous polyethyleneresin film having the crosslinked polymer supported on one side thereofin an amount of 0.5 g/m².

The negative-electrode sheet obtained in Reference Example 1, the porouspolyethylene resin film having the crosslinked polymer supportedthereon, and the positive-electrode sheet obtained in Reference Example1 were stacked in this order so that the crosslinked polymer on theporous film faced the positive-electrode sheet. Thus, anelectrode/crosslinked-polymer-supporting porous polyethylene resin filmstack was obtained. This stack was introduced into an aluminum laminatepackage, and an electrolyte solution constituted of an ethylenecarbonate/diethyl carbonate (1/1 by weight) mixed solvent containinglithium hexafluorophosphate dissolved therein in a concentration of 1.4mol/L was poured into the package, which was then sealed. Thereafter,the resultant battery was charged at a current of 0.2 CmA until thevoltage reached 3.5 V, and was then placed in a 50° C. thermostaticchamber for 24 hours to accelerate adhesion between the electrode sheetsand the porous polyethylene resin film.

Thus, a sealed laminate type battery was obtained.

With respect to the battery obtained, the results of the ratecharacteristics, continuous charge characteristics, and high-temperaturestorage test of the battery are shown in Table 1 together with thedegree of areal shrinkage of the porous substrate, as in the case ofComparative Example 1.

Reference Example 5

(Preparation of Reactive Polymer)

Into a three-necked flask having a capacity of 500 mL and equipped witha reflux condenser were introduced 98 g of methyl methacrylate, 2.0 g of4-hydroxybutyl acrylate, 25 g of ethyl acetate, and 0.20 g ofN,N′-azobisisobutyronitrile. While nitrogen gas was being introduced,the contents were stirred and mixed for 30 minutes and then heated to70° C. to initiate radical polymerization. At the time when about 2hours had passed, an increase in the viscosity of the reaction mixturewas observed. Thereafter, while ethyl acetate was being added, thetemperature was kept almost constant and the polymerization wascontinued for further 8 hours.

After completion of the reaction, the reaction mixture obtained wascooled to 40° C., and ethyl acetate was added thereto. Thereafter, thecontents were stirred and mixed until the mixture became whollyhomogeneous. Thus, a reactive-polymer solution (concentration, 25% byweight) was obtained.

Subsequently, 100 g of the polymer solution was added to 600 mL ofheptane with stirring with a high-speed mixer to precipitate thepolymer. The polymer was taken out by filtration, repeatedly washed withheptane several times, dried in air, and then vacuum-dried in adesiccator for 6 hours to obtain the reactive polymer as a white powder.

(Preparation of Polycarbonate Urethane Prepolymer Terminated byIsocyanate Group)

While nitrogen gas was being introduced into a three-necked flask havinga capacity of 300 mL and equipped with a reflux condenser, 20 g of apoly(hexamethylene carbonate) diol (the Nippollan 980R) and 20.94 g ofethyl acetate were introduced into the flask and stirred to dissolve thediol. Thereafter, 24.15 g of the same polyfunctional isocyanate asdescribed above (hexamethylene diisocyanate/trimethylolpropane adduct;ethyl acetate solution; solid content, 25%; Coronate HL, manufactured byNippon Polyurethane Co., Ltd.) was mixed with that solution. Theresultant mixture was evenly stirred and then heated to 60° C. andreacted for 15 hours. The mixture was cooled to room temperature, and151.88 g of ethyl acetate was further added thereto to obtain an ethylacetate solution of an isocyanate-terminated polycarbonate urethaneprepolymer having a concentration of 12% by weight.

Example 2

Six grams of the reactive polymer obtained in Reference Example 5 wasdissolved in ethyl acetate at room temperature to prepare 50 g of areactive-polymer solution having a concentration of 12% by weight.Thereto was added 16 g of the ethyl acetate solution of a urethaneprepolymer having a concentration of 12% by weight obtained in ReferenceExample 5. This mixture was heated to 80° C. and reacted for 20 hourswith stirring. Thereafter, the reaction mixture was cooled, and 132 g ofethyl acetate was added thereto to prepare a coating fluid having asolid concentration of 4% by weight.

This coating fluid was applied to one side of a polypropylene resinsheet with a spin coater and then dried at 50° C. for 5 minutes tovolatilize the ethyl acetate. Thus, a thin layer constituted of amixture of the reactive polymer and the urethane prepolymer was formedon the polypropylene resin sheet.

This polypropylene resin sheet was superposed on the porous polyethyleneresin film obtained in Reference Example 2, so that the thin layerconstituted of a mixture of the reactive polymer and the urethaneprepolymer was in contact with the porous film. The resultant stack waspassed through the nip between laminating rolls heated at a temperatureof 125° C. and was thereby heated and pressed to transfer the thin layerconstituted of a mixture of the reactive polymer and the urethaneprepolymer to one side of the porous polyethylene resin film.

Subsequently, the stack composed of the porous polyethylene resin filmhaving the thin layer and the polypropylene resin sheet was heated at90° C. for 48 hours to react the reactive polymer with the urethaneprepolymer and crosslink the reactive polymer, thereby forming acrosslinked polymer having an insoluble content of 98%. Thereafter, thepolypropylene resin sheet was peeled off to obtain a porous polyethyleneresin film having the crosslinked polymer supported on one side thereofin an amount of 0.5 g/m².

The negative-electrode sheet obtained in Reference Example 1, the porouspolyethylene resin film having the crosslinked polymer supportedthereon, and the positive-electrode sheet obtained in Reference Example1 were stacked in this order so that the crosslinked polymer on theporous film faced the positive-electrode sheet. Thus, anelectrode/crosslinked-polymer-supporting porous polyethylene resin filmstack was obtained. This stack was introduced into an aluminum laminatepackage, and an electrolyte solution constituted of an ethylenecarbonate/diethyl carbonate (1/1 by weight) mixed solvent containinglithium hexafluorophosphate dissolved therein in a concentration of 1.4mol/L was poured into the package, which was then sealed. Thereafter,the resultant battery was charged at a current of 0.2 CmA until thevoltage reached 3.5 V, and was then placed in a 50° C. thermostaticchamber for 24 hours to accelerate adhesion between the electrode sheetsand the porous polyethylene resin film.

Thus, a sealed laminate type battery was obtained.

With respect to the battery obtained, the results of the ratecharacteristics, continuous charge characteristics, and high-temperaturestorage test of the battery are shown in Table 1 together with thedegree of areal shrinkage of the porous substrate (porous polyethyleneresin film), as in the case of Comparative Example 1.

Example 3

At room temperature, 6.0 g of the reactive polymer obtained in ReferenceExample 5 was dissolved in ethyl acetate to prepare 50 g of areactive-polymer solution having a concentration of 12% by weight.Thereto was added 60 g of the ethyl acetate solution of a urethaneprepolymer having a concentration of 12% by weight obtained in ReferenceExample 5. This mixture was heated to 80° C. and reacted for 20 hourswith stirring. The resultant reaction mixture was cooled, and 220 g ofethyl acetate was added thereto to prepare a coating fluid having asolid concentration of 4% by weight.

This coating fluid was applied to one side of a polypropylene resinsheet with a spin coater and then dried at 50° C. for 5 minutes tovolatilize the ethyl acetate. Thus, a thin layer constituted of thereactive polymer, the urethane prepolymer, and a product of reactiontherebetween was formed on the polypropylene resin sheet.

This polypropylene resin sheet was superposed on the porous polyethyleneresin film obtained in Reference Example 2, so that the thin layer wasin contact with the porous film. The resultant stack was passed throughthe nip between laminating rolls heated at a temperature of 125° C. andwas thereby heated and pressed to transfer the thin layer to one side ofthe porous polyethylene resin film.

Subsequently, the stack composed of the porous polyethylene resin filmhaving the thin layer and the polypropylene resin sheet was heated at90° C. for 48 hours to react the reactive polymer with the urethaneprepolymer and crosslink the reactive polymer, thereby forming acrosslinked polymer having an insoluble content of 98%. Thereafter, thepolypropylene resin sheet was peeled off to obtain a porous polyethyleneresin film having the crosslinked polymer supported on one side thereofin an amount of 0.5 g/m².

The negative-electrode sheet obtained in Reference Example 1, the porouspolyethylene resin film having the crosslinked polymer supportedthereon, and the positive-electrode sheet obtained in Reference Example1 were stacked in this order so that the crosslinked polymer on theporous film faced the positive-electrode sheet. Thus, anelectrode/crosslinked-polymer-supporting porous polyethylene resin filmstack was obtained. This stack was introduced into an aluminum laminatepackage, and an electrolyte solution constituted of an ethylenecarbonate/diethyl carbonate (1/1 by weight) mixed solvent containinglithium hexafluorophosphate dissolved therein in a concentration of 1.4mol/L was poured into the package, which was then sealed. Thereafter,the resultant battery was charged at a current of 0.2 CmA until thevoltage reached 3.5 V, and was then placed in a 50° C. thermostaticchamber for 24 hours to accelerate adhesion between the electrode sheetsand the porous polyethylene resin film.

Thus, a sealed laminate type battery was obtained.

With respect to the battery obtained, the results of the ratecharacteristics, continuous charge characteristics, and high-temperaturestorage test of the battery are shown in Table 1 together with thedegree of areal shrinkage of the porous substrate (porous polyethyleneresin film), as in the case of Comparative Example 1.

Reference Example 6

(Preparation of Reactive Polymer)

Into a three-necked flask having a capacity of 500 mL and equipped witha reflux condenser were introduced 98 g of methyl methacrylate, 2.0 g of4-hydroxybutyl acrylate, 25 g of ethyl acetate, and 0.20 g ofN,N′-azobisisobutyronitrile. While nitrogen gas was being introduced,the contents were stirred and mixed for 30 minutes and then heated to70° C. to initiate radical polymerization. At the time when about 2hours had passed, an increase in the viscosity of the reaction mixturewas observed. Thereafter, while ethyl acetate was being added, thetemperature was kept almost constant and the polymerization wascontinued for further 8 hours.

After completion of the reaction, the reaction mixture obtained wascooled to 40° C., and ethyl acetate was added thereto. Thereafter, thecontents were stirred and mixed until the mixture became whollyhomogeneous. Thus, a reactive-polymer solution (concentration, 25% byweight) was obtained.

Subsequently, 100 g of the polymer solution was added to 600 mL ofheptane with stirring with a high-speed mixer to precipitate thepolymer. The polymer was taken out by filtration, repeatedly washed withheptane several times, dried in air, and then vacuum-dried in adesiccator for 6 hours to obtain the reactive polymer as a white powder.

(Preparation of Polycarbonate Urethane Prepolymer Terminated byIsocyanate Group)

While nitrogen gas was being introduced into a three-necked flask havinga capacity of 300 mL and equipped with a reflux condenser, 18 g of apoly(alkylene carbonate) diol having, in the molecule, alkylene chainsdiffering in the number of carbon atoms (Duranol G3452, manufactured byAsahi Kasei Chemicals Corporation) and 41.68 g of toluene wereintroduced into the flask and stirred to dissolve the diol. Thereafter,9.79 g of a polyfunctional isocyanate (Coronate 2770, manufactured byNippon Polyurethane Co., Ltd.) was mixed with the solution. Theresultant mixture was evenly stirred and mixed and was then heated to60° C. and reacted for 15 hours. The mixture was cooled to roomtemperature, and 152.83 g of toluene was further added thereto to obtaina urethane prepolymer solution having a concentration of 12.5% byweight.

Example 4

Nine grams of the reactive polymer obtained in Reference Example 6 wasdissolved in toluene at room temperature to prepare 90 g of areactive-polymer solution having a concentration of 10% by weight. Thissolution was mixed with 32.4 g of the urethane prepolymer solutionhaving a concentration of 12.5% by weight, and this mixture was stirred.Furthermore, 95.1 g of toluene was added to the resultant mixturesolution to prepare a coating fluid having a solid concentration of 6%by weight.

This coating fluid was applied to one side of a polypropylene resinsheet with a spin coater and then dried at 50° C. for 5 minutes tovolatilize the toluene. Thus, a thin layer constituted of a mixture ofthe reactive polymer and the urethane prepolymer was formed on thepolypropylene resin sheet.

This polypropylene resin sheet was superposed on the porous polyethyleneresin film obtained in Reference Example 2, so that the thin layerconstituted of a mixture of the reactive polymer and the urethaneprepolymer was in contact with the porous film. The resultant stack waspassed through the nip between laminating rolls heated at a temperatureof 125° C. and was thereby heated and pressed to transfer the thin layerconstituted of a mixture of the reactive polymer and the urethaneprepolymer to one side of the porous polyethylene resin film.

Subsequently, the stack composed of the porous polyethylene resin filmhaving the thin layer and the polypropylene resin sheet was heated at90° C. for 48 hours to react the reactive polymer with the urethaneprepolymer and crosslink the reactive polymer, thereby forming acrosslinked polymer having an insoluble content of 98%. Thereafter, thepolypropylene resin sheet was peeled off to obtain a porous polyethyleneresin film having the crosslinked polymer supported on one side thereofin an amount of 0.5 g/m².

The negative-electrode sheet obtained in Reference Example 1, the porouspolyethylene resin film having the crosslinked polymer supportedthereon, and the positive-electrode sheet obtained in Reference Example1 were stacked in this order so that the crosslinked polymer on theporous film faced the positive-electrode sheet. Thus, anelectrode/crosslinked-polymer-supporting porous polyethylene resin filmstack was obtained. This stack was introduced into an aluminum laminatepackage, and an electrolyte solution constituted of an ethylenecarbonate/diethyl carbonate (1/1 by weight) mixed solvent containinglithium hexafluorophosphate dissolved therein in a concentration of 1.4mol/L was poured into the package, which was then sealed. Thereafter,the resultant battery was charged at a current of 0.2 CmA until thevoltage reached 3.5 V, and was then placed in a 50° C. thermostaticchamber for 24 hours to accelerate adhesion between the electrode sheetsand the porous polyethylene resin film.

Thus, a sealed laminate type battery was obtained.

With respect to the battery obtained, the results of the ratecharacteristics, continuous charge characteristics, and high-temperaturestorage test of the battery are shown in Table 1 together with thedegree of areal shrinkage of the porous substrate (porous polyethyleneresin film), as in the case of Comparative Example 1.

Reference Example 7

(Preparation of Reactive Polymer)

Into a three-necked flask having a capacity of 500 mL and equipped witha reflux condenser were introduced 98 g of methyl methacrylate, 2.0 g of4-hydroxybutyl acrylate, 25 g of ethyl acetate, and 0.20 g ofN,N′-azobisisobutyronitrile. While nitrogen gas was being introduced,the contents were stirred and mixed for 30 minutes and then heated to70° C. to initiate radical polymerization. At the time when about 2hours had passed, an increase in the viscosity of the reaction mixturewas observed. Thereafter, while ethyl acetate was being added, thetemperature was kept almost constant and the polymerization wascontinued for further 8 hours.

After completion of the reaction, the reaction mixture obtained wascooled to 40° C., and ethyl acetate was added thereto. Thereafter, thecontents were stirred and mixed until the mixture became whollyhomogeneous. Thus, a reactive-polymer solution (concentration, 25% byweight) was obtained.

Subsequently, 100 g of the polymer solution was added to 600 mL ofheptane with stirring with a high-speed mixer to precipitate thepolymer. The polymer was taken out by filtration, repeatedly washed withheptane several times, dried in air, and then vacuum-dried in adesiccator for 6 hours to obtain the reactive polymer as a white powder.

(Preparation of Polycarbonate Urethane Prepolymer Terminated byIsocyanate Group)

While nitrogen gas was being introduced into a three-necked flask havinga capacity of 300 mL and equipped with a reflux condenser, 18 g of apoly(alkylene carbonate) diol having, in the molecule, alkylene chainsdiffering in the number of carbon atoms (Duranol G3452, manufactured byAsahi Kasei Chemicals Corporation) and 39.23 g of toluene wereintroduced into the flask and stirred to dissolve the diol. Thereafter,8.16 g of a polyfunctional isocyanate (Coronate 2770, manufactured byNippon Polyurethane Co., Ltd.) was mixed with the solution. Theresultant mixture was evenly stirred and mixed and was then heated to60° C. and reacted for 15 hours. The mixture was cooled to roomtemperature, and 143.86 g of toluene was further added thereto to obtaina urethane prepolymer solution having a concentration of 12.5% byweight.

Example 5

Six grams of the reactive polymer obtained in Reference Example 7 wasdissolved in toluene at room temperature to prepare 60 g of areactive-polymer solution having a concentration of 10% by weight. Thissolution was mixed with 48 g of the urethane prepolymer solution havinga concentration of 12.5% by weight, and this mixture was stirred.Furthermore, 92 g of toluene was added to the resultant mixture solutionto prepare a coating fluid having a solid concentration of 6% by weight.

This coating fluid was applied to one side of a polypropylene resinsheet with a spin coater and then dried at 50° C. for 5 minutes tovolatilize the toluene. Thus, a thin layer constituted of a mixture ofthe reactive polymer and the urethane prepolymer was formed on thepolypropylene resin sheet.

This polypropylene resin sheet was superposed on the porous polyethyleneresin film obtained in Reference Example 2, so that the thin layerconstituted of a mixture of the reactive polymer and the urethaneprepolymer was in contact with the porous film. The resultant stack waspassed through the nip between laminating rolls heated at a temperatureof 125° C. and was thereby heated and pressed to transfer the thin layerconstituted of a mixture of the reactive polymer and the urethaneprepolymer to one side of the porous polyethylene resin film.

Subsequently, the stack composed of the porous polyethylene resin filmhaving the thin layer and the polypropylene resin sheet was heated at90° C. for 48 hours to react the reactive polymer with the urethaneprepolymer and crosslink the reactive polymer, thereby forming acrosslinked polymer having an insoluble content of 98%. Thereafter, thepolypropylene resin sheet was peeled off to obtain a porous polyethyleneresin film having the crosslinked polymer supported on one side thereofin an amount of 0.5 g/m².

The negative-electrode sheet obtained in Reference Example 1, the porouspolyethylene resin film having the crosslinked polymer supportedthereon, and the positive-electrode sheet obtained in Reference Example1 were stacked in this order so that the crosslinked polymer on theporous film faced the positive-electrode sheet. Thus, anelectrode/crosslinked-polymer-supporting porous polyethylene resin filmstack was obtained. This stack was introduced into an aluminum laminatepackage, and an electrolyte solution constituted of an ethylenecarbonate/diethyl carbonate (1/1 by weight) mixed solvent containinglithium hexafluorophosphate dissolved therein in a concentration of 1.4mol/L was poured into the package, which was then sealed. Thereafter,the resultant battery was charged at a current of 0.2 CmA until thevoltage reached 3.5 V, and was then placed in a 50° C. thermostaticchamber for 24 hours to accelerate adhesion between the electrode sheetsand the porous polyethylene resin film.

Thus, a sealed laminate type battery was obtained.

With respect to the battery obtained, the results of the ratecharacteristics, continuous charge characteristics, and high-temperaturestorage test of the battery are shown in Table 1 together with thedegree of areal shrinkage of the porous substrate (porous polyethyleneresin film), as in the case of Comparative Example 1.

Referential Example 1

A porous polytetrafluoroethylene resin film having a porosity of 97% anda thickness of 5 μm was superposed on and supported on one side of theporous polyethylene resin film obtained in Reference Example 2.

The negative-electrode sheet obtained in Reference Example 1, the porouspolyethylene resin film having the porous polytetrafluoroethylene resinfilm supported thereon, and the positive-electrode sheet obtained inReference Example 1 were stacked in this order so that the porousfluororesin film on the porous polyethylene resin film faced thepositive-electrode sheet. Thus, anelectrode/porous-fluororesin-film-supporting porous polyethylene resinfilm stack was obtained. This stack was introduced into an aluminumlaminate package, and an electrolyte solution constituted of an ethylenecarbonate/diethyl carbonate (1/1 by weight) mixed solvent containinglithium hexafluorophosphate dissolved therein in a concentration of 1.4mol/L was poured into the package, which was then sealed. Thereafter,the resultant battery was charged at a current of 0.20 CmA until thevoltage reached 3.5 V. Thus, a sealed laminate type battery wasobtained.

The battery thus obtained was evaluated for continuous chargecharacteristics and high-temperature storability in the same manners asin Comparative Example 1. As apparent from the results given in Table 1,no increase in current value was observed in the continuous charge. Thebattery after the high-temperature storage had a voltage of 4.1 V.

TABLE 1 Continuous Insol- Rate charge High- Degree uble charac- charac-temperature of areal content teristics teristics storability shrinkage(%) (%) (h) (V) (%) Comparative — 88 65 3.78 73 Example 1 Comparative —80 70 3.95 6 Example 2 Example 1 99 85 no 4.09 13 increase Example 2 9879 no 4.10 9 increase Example 3 98 83 no 4.11 12 increase Example 4 9876 no 4.10 8 increase Example 5 98 81 no 4.11 8 increase Referential — —no 4.10 — Example 1 increase

As apparent from the results given in Table 1, the results of Examples 1to 5 are substantially equal to the results of Referential Example 1, inwhich a porous fluororesin film was interposed between thepositive-electrode sheet and the porous polyethylene resin film. Namely,the crosslinked polymer in the invention has oxidation resistance whichis substantially equal to that of the porous fluororesin film used inReferential Example 1.

As demonstrated above, in the battery according to the invention, thecrosslinked polymer having a polycarbonate urethane framework andinterposed between the positive electrode and the separator does notoxidatively deteriorate despite the high oxidation reactivity of thepositive electrode. Consequently, the battery suffers neitherdeterioration of the electrode/separator adhesion nor the deteriorationof battery characteristics caused by products of decomposition of thecrosslinked polymer. In addition, the crosslinked polymer functions alsoas a protective layer for the separator. Consequently, even when theporous polyolefin resin film is used as separators, these separators canbe prevented from oxidatively deteriorating.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

This application is based on a Japanese patent application No.2008-094392 filed on Mar. 31, 2008, the entire contents thereof beingherein incorporated by reference.

Further, all references cited herein are incorporated by reference intheir entirety.

Industrial Applicability

According to the invention, a battery separator having excellentoxidation resistance and having the property of adhering to an electrodeis obtained by supporting on a porous substrate a layer of a crosslinkedpolymer obtained by reacting a reactive polymer having in the moleculethereof a reactive group containing active hydrogen with a polycarbonateurethane prepolymer terminated by an isocyanate group. Furthermore, abattery using such a battery separator is provided according to theinvention.

The invention claimed is:
 1. A battery separator comprising: a poroussubstrate; and a layer of a crosslinked polymer supported on at leastone surface of the porous substrate, wherein the crosslinked polymer isobtained by reacting (a) a reactive polymer having a reactive groupcontaining a hydroxy group with (b) a polycarbonate urethane prepolymerin which two ends thereof are terminated by an isocyanate group, thepolycarbonate urethane prepolymer is obtained by reacting an aliphaticpolycarbonate diol having hydroxy groups with a polyfunctionalisocyanate having isocyanate groups, wherein a molar ratio of theisocyanate groups possessed by the polyfunctional isocyanate to thehydroxy groups possessed by the polycarbonate diol is in a range of 1.2to 3.3.
 2. The battery separator according to claim 1, wherein theporous substrate is a porous polyolefin resin film.
 3. The batteryseparator according to claim 2, wherein the porous polyolefin resin filmis a porous polyethylene resin film.
 4. An electrode/separator laminatecomprising: the separator according to claim 1; and a positive electrodeand a negative electrode laminated together with the separatorinterposed therebetween, wherein at least one of the positive electrodeand the negative electrode is adhered to the porous substrate by thecrosslinked polymer.
 5. A battery comprising the electrode/separatorlaminate according to claim
 4. 6. The battery according to claim 5,wherein the battery further comprises a nonaqueous electrolyte solutionand the layer of the crosslinked polymer faces at least the positiveelectrode.
 7. A process for producing a battery, said processcomprising: stacking a positive electrode and a negative electrodetogether with the separator according to claim 1 interposedtherebetween; introducing the resultant stack into a battery container,followed by pouring a nonaqueous electrolyte solution into the batterycontainer; and forming an electrode/separator laminate in which at leastone of the positive electrode and the negative electrode is adhered tothe porous substrate by the crosslinked polymer.
 8. The process forproducing a battery according to claim 7, wherein the positive electrodeand the negative electrode are stacked together with the separatorinterposed therebetween so that the layer of the crosslinked polymerfaces at least the positive electrode.